If a similar quake struck today, almost 40 percent of San Francisco's buildings would likely be destroyed, fires would again rage, bridges would fail, and the water supply could be completely severed, experts say.

Bay Area residents and authorities may have been lulled into a false sense of security, because the last century has been relatively quiet for California, seismologically speaking.

At least 3,000 people were killed in the wake of the 1906 quake. Some 225,000 were left homeless, out of a population of 400,000.

Just after five o'clock in the morning on April 18, residents were awoken by a strong jolt. Within seconds buildings were shaking violently, and people were thrown to the floor. But the worst was still to come. (See our time line of the great quake.)

Within minutes of the initial shocks fires began to break out across the citystoked by ruptured gas pipes, sparks from broken power cables, and unseasonably warm temperatures.

Add to this a lack of water, due to smashed water mains, and mostly wooden buildings, and you had the ingredients for catastrophe.

What If?

Without a doubt another massive earthquake is waiting in the wings, but this time it will rattle a city with nearly twice the population it had in 1906.